Reaction Dynamics in Astrochemistry: Low-Temperature … · 2018-12-05 · chemistry, aromaticity,...

PC66CH03-Kaiser ARI 28 February 2015 11:6

Reaction Dynamics inAstrochemistry:Low-Temperature Pathwaysto Polycyclic AromaticHydrocarbons in theInterstellar MediumRalf I. Kaiser,1 Dorian S.N. Parker,1

and Alexander M. Mebel21Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822;email: [email protected] of Chemistry and Biochemistry, Florida International University, Miami,Florida 33199

Annu. Rev. Phys. Chem. 2015. 66:43–67

First published online as a Review in Advance onNovember 20, 2014

The Annual Review of Physical Chemistry is online atphyschem.annualreviews.org

This article’s doi:10.1146/annurev-physchem-040214-121502

Copyright c© 2015 by Annual Reviews.All rights reserved

Keywords

cold molecular clouds, neutral-neutral reactions, nonequilibriumchemistry, aromaticity, resonantly stabilized free radical, ISM

Abstract

Bimolecular reactions of phenyl-type radicals with the C4 and C5 hydro-carbons vinylacetylene and (methyl-substituted) 1,3-butadiene have beenfound to synthesize polycyclic aromatic hydrocarbons (PAHs) with naph-thalene and 1,4-dihydronaphthalene cores in exoergic and entrance barri-erless reactions under single-collision conditions. The reaction mechanisminvolves the initial formation of a van der Waals complex and addition ofa phenyl-type radical to the C1 position of a vinyl-type group through asubmerged barrier. Investigations suggest that in the hydrocarbon reactant,the vinyl-type group must be in conjugation with a –C≡CH or –HC=CH2

group to form a resonantly stabilized free radical intermediate, which even-tually isomerizes to a cyclic intermediate followed by hydrogen loss andaromatization (PAH formation). The vinylacetylene-mediated formation ofPAHs might be expanded to more complex PAHs, such as anthracene andphenanthrene, in cold molecular clouds via barrierless reactions involvingphenyl-type radicals, such as naphthyl, which cannot be accounted for bythe classical hydrogen abstraction–acetylene addition mechanism.

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1. INTRODUCTION

Ever since the first postulation of polycyclic aromatic hydrocarbons (PAHs)—organic moleculescarrying fused benzene rings—together with their (partially) (de)hydrogenated, ionized, proto-nated, and substituted counterparts as the missing link between small carbon clusters, fullerenes,and carbonaceous nanoparticles (interstellar grains) (1), an intimate understanding of their for-mation mechanisms in the interstellar medium (ISM) has been sought from the astrophysics,astrobiology, and astrochemistry communities (2–5). Today, PAH-like molecules are presumedto be ubiquitous in the ISM (6–8), account for 10–30% of the galactic interstellar carbon (9, 10),and provide critical nucleation sites for the formation of carbonaceous dust particles (11, 12). PAHshave also been linked to the unidentified infrared (UIR) emission bands observed in the range of3–14 μm (3,300–700 cm−1) (13, 14) and to the diffuse interstellar bands (15–17), which are discreteabsorption features superimposed on the interstellar extinction curve ranging from the blue partof the visible (400 nm) to the near IR (1.2 μm). The UIR emission features and diffuse interstellarbands are not restricted to our own galaxy as UIR bands have also been observed toward the CigarGalaxy, M82 (18); a persistent debate has emerged on the role of aliphatic versus aromatic carbon-hydrogen stretches describing the 3.4-μm (2,941-cm−1) feature with methyl-substituted PAHsas potential candidates (19–21). Finally, PAH-like molecules are relevant to the astrobiologicalevolution of the ISM. In PAHs, the methylidyne (CH) moiety can be replaced by an isoelec-tronic nitrogen atom, forming biorelevant molecules such as nucleobases—key building blocksin RNA.

Despite the importance of PAH-like molecules in the astrochemical and astrobiologicalevolution of the ISM, the routes to interstellar PAHs have not been well characterized. Thediscovery of the simplest PAH, naphthalene (C10H8), together with more complex PAHs—anthracene/phenanthrene (C14H10) and pyrene/fluoranthrene (C16H10), even up to seven-ring(C28H16) species along with their C1, C2, and C3 alkyl-substituted PAHs—in at least 20 carbona-ceous chondrites strongly implies an interstellar origin (22, 23). Sophisticated isotopic studies (e.g.,on D, 13C) verified an interstellar source and proposed inner envelopes of carbon-rich asymptoticgiant branch (AGB) stars, such as IRC+10216, where temperatures can reach up to a few thou-sand Kelvin, as the key breeding grounds for these increasingly complex organic molecules (24).Because of the combustion-relevant temperatures of circumstellar envelopes, the most favorableroutes to form polycyclic aromatic species have been borrowed from the combustion chemistrycommunity. These pathways have been suggested to involve molecular weight growth processesthrough sequential reactions of aromatic radicals (ARs) and resonantly stabilized free radicals(RSFRs), such as the phenyl (C6H5) and propargyl radicals (C3H3), respectively, eventually lead-ing to carbonaceous nanoparticles. Along with acetylene (C2H2), these pathways have been con-templated as the basis for the hydrogen abstraction–acetylene addition (HACA) (25–27), phenyladdition–cyclization (28, 29), and ethynyl addition (30) mechanisms. Owing to their stability evenat elevated temperatures of several thousand Kelvin, RSFRs and ARs can reach high concentra-tions in circumstellar environments; these high concentrations make them important reactionintermediates involved in mass growth processes ultimately forming PAHs.

In recent years, based on current models, it has become apparent that PAHs are destroyed fasterin the ISM than they can be synthesized (31–33). First, theoretical studies, driven by laboratorystudies on the loss of acetylene (C2H2) upon the photolysis of small PAHs, predicted lifetimes of afew hundred million years for PAHs in the diffuse ISM. Second, interstellar shock waves driven bysupernova explosions forecast similar short lifetimes for PAHs in the ISM (34). Finally, energeticcosmic ray bombardment rapidly degrades small PAHs, once again on a timescale of only a fewhundred million years. These timescales are much shorter than the timescale for the injection

44 Kaiser · Parker · Mebel

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of new, PAH-based material into the ISM by carbon-rich AGB stars together with carbon-richplanetary nebulae as the descendants of AGB stars (i.e., around two billion years) (34). Finally, arecent computational study by Mebel and colleagues (35) revealed that the HACA mechanism—thecentral backbone in contemporary PAH synthesis in circumstellar envelopes of carbon-rich AGBstars—terminates with the formation of naphthalene and acenaphthalene (C12H8). More complexPAHs, even those with only three six-membered rings (anthracene, phenanthrene), cannot beformed via HACA-based pathways. Consequently, the ubiquitous presence of PAHs in the ISMand in carbonaceous chondrites presents a mystery and implies a crucial, hitherto unexplainedroute to the fast chemical growth of PAHs in the cold environment of the ISM at temperaturesas low as 10 K.

To fill the void of rapid and facile formation routes to PAHs, scientists have often looked toion-molecule reactions as a one-size-fits-all route. The idea of an ion-molecule-dominated in-terstellar chemistry originated from the 1970s, when the molecular complexity of the ISM washighly underestimated and astrochemists had the misconception that ion-molecule reactions are al-ways barrierless and all neutral-neutral reactions possess entrance barriers. Current astrochemicalmodels speculate that the synthesis of benzene and more complex PAHs is dictated by convolutednetworks of ion-molecule reactions involving, for instance, methane (CH4), ethylene (C2H4), andpropargyl with C2H+

5 and C4H+2 ions to initially form C6H+

5 ions (of an unknown structure) (36,37), which then further react to form PAH-like cations via multistep pathways (38). However, thevalidity of these routes of PAH formation has remained conjectural, as the outcomes of these bi-molecular reactions have often been educationally guessed and not a single laboratory experimentcould corroborate the extent to which PAHs are formed in these elementary reactions. Upon theremoval of unstudied ion-molecule reactions leading to C6H+

5 , for instance, the peak abundance ofC6H+

5 drops by over three orders of magnitude, resulting in a similar reduction of benzene formedvia ion-molecule reactions. Combined with the uncertainties in the assumed rate constant of theassociation reaction of C6H+

5 with molecular hydrogen to form C6H+7 (39), not even the benzene

molecule—the key building block of all PAHs—could be formed in sufficiently high quantities(40). However, the incorporation of barrierless and rapid neutral-neutral reactions involving reac-tions of ethynyl radicals (C2H) with 1,3-butadiene (C4H6; H2CCHCHCH2) emerged as the mostprominent route to synthesize the benzene molecule in cold molecular clouds such as TMC-1,holding temperatures as low as 10 K. These findings also rationalize the detection of benzenewith the Infrared Space Observatory toward the protoplanetary nebula CRL 618, with benzeneapproximately 40 times less abundant than acetylene (41), and suggest the ubiquitous presenceof the benzene molecule throughout the ISM. Successive studies of bimolecular neutral-neutralreactions of ethynyl and dicarbon (C2) with vinylacetylene (C4H4), 1,3-butadiene, and isoprene(2-methyl-1,3-butadiene; C5H8), exploiting the crossed molecular beam technique and combin-ing reactive scattering data with electronic structure calculations, provided compelling evidencethat monocyclic aromatic molecules can be formed via exoergic and fast, barrierless reactionsfrom acyclic precursors as a result of a single collision (Figure 1). These investigations deliveredthe first solid evidence that bimolecular cyclization reactions involving two neutral reactants cansynthesize monocyclic aromatic molecules in the gas phase.

This review expands the novel concept of aromatization reactions via bimolecular neutral-neutral collisions from monocyclic systems to PAHs. It compiles the current knowledge on theformation of naphthalene—the simplest PAH—with its derivatives (e.g., hydrogenated naph-thalene, mono- and di-methyl-substituted isomers) involving exoergic and fast, barrierless reac-tions of phenyl-type radicals [e.g., phenyl, meta-tolyl (m-C6H4CH3), para-tolyl ( p-C6H4CH3)]with the C4 and C5 hydrocarbons vinylacetylene, 1,3-butadiene, 1-methyl-1,3-butadiene (C5H8),and 2-methyl-1,3-butadiene (C5H8) as extracted from crossed molecular beam reactions and

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1 2 3

CH3 CH2

4 5

Figure 1(Top row) Monocyclic aromatic molecules formed in exoergic and barrierless bimolecular reactions fromethynyl (C2H) with 1,3-butadiene (C4H6) [benzene; C6H6 (1)] (40), dicarbon (C2) with 1,3-butadiene[phenyl; C6H5 (2)] (42), and ethynyl with vinylacetylene (C4H4) [o-benzyne; C6H4 (3)] (43). (Bottom row)Substituted monocyclic aromatic molecules formed in exoergic and barrierless bimolecular reactionsbetween ethynyl and 2-methyl-1,3-butadiene (C5H8) [toluene; C6H5CH3 (4)] (44) and dicarbon with2-methyl-1,3-butadiene [benzyl; C6H5CH2 (5)] (45).

electronic structure calculations. These systems are prototypical representatives of a new classof reactions forming PAHs and their derivatives involving an initial van der Waals complex anda submerged barrier in strongly exoergic processes in the gas phase. These characteristics makebimolecular collisions of phenyl-type radicals with unsaturated C4 and C5 hydrocarbons uniquecandidates to synthesize (methyl-substituted) PAHs and their partially hydrogenated counterpartsin cold molecular clouds at temperatures as low as 10 K, bringing us closer to understanding theformation mechanisms of PAHs in ultracold interstellar environments.

2. THE CROSSED MOLECULAR BEAM APPROACH

Which experimental approach can best reveal the chemical dynamics involved in the formationof PAHs in the gas phase? Experiments conducted under single-collision conditions, in whichparticles A of one supersonic beam are made to collide only with particles BC of a second beam(Equation 1), are ideal to investigate the chemical dynamics of bimolecular gas phase reactions atthe most fundamental, microscopic level (46–58). These are crossed molecular beam experiments.In contrast to bulk experiments, in which reactants are mixed, the crossed beam approach has theunique capability of generating highly reactive radicals, such as phenyl-type radicals (AR.) andhydrocarbons (RH) in separate pulsed supersonic beams (Equation 2). In principle, both reactantbeams can be prepared in well-defined quantum states before they cross at a specific collisionenergy under single-collision conditions. These features provide an unprecedented opportunity toobserve the consequences of a single-collision event and to identify the nascent reaction products,


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excluding secondary collisions and wall effects, ultimately gaining information on the chemicaldynamics of the reaction:

A + BC → AB + C, (1)

AR + RH → PAH + H. (2)

Over the past few decades, the use of crossed molecular beams has led to an unprecedentedadvancement in our understanding of fundamental principles underlying chemical reactivity. De-tailed experimental studies of simple three-atom reactions established experimental benchmarks,such as the reactions of chlorine (59, 60), fluorine (61, 62), deuterium (63, 64), carbon (65), nitrogen(58, 66), oxygen (67–69), and sulfur atoms (70) with molecular hydrogen. This approach has beenextended to four-atom [OH/CO (71), OH/H2 (72), CN/H2 (73)], five-atom [C/C2H2 (74)], andsix-atom systems [Cl/CH4 (75), O/CHD3(76), F/CD4 (77)]. These systems are prototypical re-actions in bridging our theoretical understanding of reactive scattering via dynamics calculationson chemically accurate potential energy surfaces with experimental observations (78). Althoughinterest in these simple elementary reactions still continues, with the development of powerfultheoretical methods, attention has turned in recent years to more complex systems of signifi-cant interest in combustion processes and catalysis, as well as interstellar and planetary chemistry(46–58, 79, 80). However, only recently have there been experimental studies of the reactions ofphenyl-type radicals with hydrocarbon molecules leading to the formation of PAH-like speciesunder single-collision conditions. This delay resulted from previously insurmountable difficul-ties of generating stable supersonic beams of phenyl-type radical reactants with sufficiently highconcentrations to detect the product molecules.

The formation of PAHs and their hydrogenated as well as methyl-substituted counterpartswas explored systematically under single-collision conditions as provided in an ultraclean crossedbeam machine (46–50, 52). The main chamber consisted of a stainless steel box evacuated bymagnetically suspended turbo molecular pumps to the low 10−8 Torr region. Two source chambersfor separate supersonic beams were located inside the main chamber; in its current geometry,the beams cross perpendicularly. In the primary source, a pulsed supersonic beam of helium-seeded phenyl, meta-tolyl, or para-tolyl radicals was prepared by photolyzing the appropriatechlorine-substituted precursor molecule [i.e., chlorobenzene (C6H5Cl), meta-chlorotoluene (m-ClC6H4CH3), and para-chlorotoluene ( p-ClC6H4CH3)] at 193 nm. The radical beam passedthrough a skimmer into the main chamber; a chopper wheel located after the skimmer and priorto the collision center selected slices of the radical beam with well-defined peak velocities tunablebetween 1,600 and 1,800 ms−1, which then reached the interaction region with typical numberdensities of approximately ∼3 × 1013 radicals per cubed centimeter. Each beam was so dilute thatcollisions within the molecular beam were negligible but were sufficiently dense to monitor theproducts. This section of the radical beam then intersected a pulsed hydrocarbon beam under well-defined collision energies up to 55 kJ mol−1. Importantly, the incorporation of pulsed supersonicbeams is key in untangling the reaction dynamics of PAH formation. First, pulsed UV lasers canbe operated in concert with pulsed beams to selectively photolyze the precursor molecule in themolecular beam to form the desired radical reactant. Second, the operation of pulsed valves allowsreactions with often-expensive (partially) deuterated chemicals to be carried out; this is crucialto extract key information on the reaction dynamics, such as the position of the hydrogen ordeuterium loss(es) if multiple reaction pathways are involved.

Which detection scheme is incorporated for the reaction products? Spectroscopic detectionschemes such as laser-induced fluorescence and Rydberg tagging are restricted to hydrogen,


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deuterium, and oxygen atoms and to species such as hydroxyl radicals (OH) (i.e., those withwell-established spectroscopic fingerprints) (81). Therefore, this approach is not suitable for thedetection of PAH-like species, whose a priori spectroscopic properties are, with the exception ofnaphthalene, unknown. Similarly, it is problematic to identify PAH-type molecules via ion imagingand tunable synchrotron radiation because absolute photoionization cross sections to determinebranching ratios do not exist. Therefore, the ultraclean crossed beam machine incorporates a triplydifferentially pumped, universal quadrupole mass spectrometric detector coupled to an electronimpact ionizer operating at extreme ultrahigh vacuum conditions down to the medium 10−13 Torrrange, which is rotatable within the scattering plane as defined by both beams. Here, any speciesreactively scattered from the collision center after a single-collision event can be ionized in theelectron impact ionizer and in principle can have the mass (and the molecular formula) of allproducts of a bimolecular reaction determined by varying the mass-to-charge ratio (m/z) in themass filter. Because of the universal electron impact ionization of the products, even species withunknown spectroscopic properties can be detected. In our setup, we are operating the ionizer atelectron energies of 80 eV at an energy at which the ionization cross sections of organic moleculesare at their maximum. Our ionizer can also be operated via soft electron impact ionization aspioneered by Casavecchia et al. (57, 58, 80, 82). This approach utilizes electrons with low, tunableenergy (8–30 eV) to reduce or even eliminate potential complications from dissociative ionization.However, in the case of the present experiments, soft ionization has one disadvantage: At electronenergies of 8–30 eV, the ionization cross sections of the newly formed molecules are at least afactor of 20 lower than the electron impact ionization cross sections with 80-eV electrons (83).Therefore, in the case of pulsed crossed beam experiments with a lower duty cycle compared tocontinuous sources, soft ionization is impractical. However, soft electron impact ionization wasutilized to characterize the primary reactant beams on axis and in situ.

Which observables of key relevance to PAH formation can be extracted from these crossedmolecular beam experiments? First, mass spectrometry allows for the determination of the massand hence the molecular formula of the products of a bimolecular reaction by varying m/zin the mass filter. Second, by systematically exploiting (partially) isotopically labeled reactantmolecules, most commonly partially deuterated reactants, one can trace the position of thehydrogen or deuterium loss. For instance, in the reaction of ethynyl radicals with methylacetylene(CH3CCH), the atomic hydrogen loss leads to the formation of product(s) with a molecularformula C5H4 (64 u). However, we cannot determine if the hydrogen atom is lost from the formerethynyl radical, from the methyl group, or from the acetylenic group. Nevertheless, potentialatomic hydrogen losses in the crossed beam studies of the C2H-CD3CCD, C2D-CH3CCD, andC2D-CD3CCH systems enable us to identify the position(s) of the atomic hydrogen loss bytracking the production of ions at the masses corresponding to the isotopologs C5D4, C5H2D2,and C5D4 at m/z = 68, 66, and 68, respectively (84–86). Third, because the triply differentiallypumped detector is rotatable within the plane defined by both beams, it is possible to map out, atdistinct m/z, the laboratory angular distribution and the time of flight spectra of the products fromthe interaction region over a finite flight distance. By accounting for the apparatus functions as wellas angular spreads, a forward-convolution technique transforms the data from the laboratory tothe center-of-mass reference frame. These center-of-mass functions [translational energy, P(ET ),and angular distributions, T(θ )] are crucial to untangle the chemical dynamics of the reaction andto identify the nascent reaction products under single-collision conditions. The output of thisprocess is the generation of a product flux contour map, I(θ , u) = P(u) × T(θ ), which reports theflux of the reactively scattered products (I) as a function of the center-of-mass scattering angle(θ ) and product velocity (u). This function is defined as the reactive differential cross section andcan be seen as the image of the chemical reaction containing all the information on the scattering


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process. Neither bulk nor kinetic experiments can supply this information. These functionscontain some basic information to untangle the underlying chemical dynamics of the reaction.

Considering the center-of-mass translational energy distribution, P(ET ), the maximum transla-tional energy of the products (Emax) can be exploited to obtain the reaction energy. Based on energyconversation, for those products born without internal excitation, subtracting the collision energyfrom the maximum translational energy of the products yields the exoergicity of the reaction. Thisis crucially important when different structural isomers with different reaction exoergicities areproduced. As discussed above, in the case of polyatomic reactions, it is always useful to conduct thereactions systematically with (partially) deuterated reactants, as multiple structural isomers can beformed. Therefore, by comparing the experimental reaction energies to form (partially) deuter-ated products at distinct m/z with theoretically computed reaction energies, we can elucidate theproduct isomers formed together with their branching ratios (87–89). Additionally, in the most fa-vorable case, the distribution maximum of P(ET ) can provide the order of magnitude of the barrierheight in the exit channel (87–89). If P(ET ) peaks at zero or close to zero translational energy, thebond rupture within the decomposing reaction intermediate has either no or only a small exit bar-rier (loose exit transition state). Conversely, P(ET ) can depict pronounced maxima away from zerotranslational energy, suggesting tight exit transition states and a significant geometry rearrange-ment, as well as a significant electron density change from the decomposing intermediate to theproducts resulting in a repulsive bond rupture. The center-of-mass angular distribution [T(θ )]provides information on the reaction mechanism (e.g., direct, indirect, osculating) and might helpin estimating the lifetime of the decomposing complex. By comparing the center-of-mass func-tions with potential energy surfaces and the geometries of reactants, products, and intermediates,one can also infer the reaction intermediate(s) and the nature of the decomposing complex(es).This method can provide direct insight into the nature of the chemical reaction and, in some cases,the preferential rotational axis of the fragmenting complex(es) and the disposal of excess energyinto the products’ internal degrees of freedom as a function of the scattering angle and collisionenergy (48). With respect to PAH formation, indirect reactions proceeding via complex formationare critically important. These reactions depict reactive scattering flux over the complete angularrange. If the flux contour map portrays a symmetric profile around 90◦, the forward-backwardsymmetry is characteristic of a bimolecular reaction proceeding through a reaction intermediateholding a lifetime longer than its rotation period. The complex lives long enough on the picosec-ond timescale to rotate several times, and it will have forgotten the directions of the incomingcollision partners. The decomposing complex can also have geometrical constraints. For instance,a forward-backward symmetric function, which yields a distribution maximum at 90◦, results froma reaction intermediate, which loses a hydrogen atom perpendicularly to the rotational plane of thefragmenting intermediate almost parallel to the total angular momentum vector. Furthermore,the angular flux distribution can be asymmetric around 90◦; often the flux at 0◦ is larger than thatat 180◦. In this simple scenario, a so-called osculating complex exists: The reaction is still indirectand proceeds via the formation of an intermediate, but the lifetime of the latter is on the order ofthe rotation period, and multiple rotations cannot occur. Here, the asymmetry can be used as aclock to estimate the lifetime of the intermediate.

3. RESULTS

3.1. Naphthalene

The formation of naphthalene—the prototypical PAH with two six-membered rings—has beenexplored via the bimolecular reactions of the phenyl radical with vinylacetylene and the D5-phenyl


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6CH3

H3C

87

9

H3C

CH3

10 11CH3

CH3

1312

Figure 2Bicyclic aromatic molecules and their methyl-substituted counterparts formed in exoergic and de factobarrierless bimolecular reactions of phenyl-type radicals with vinylacetylene (C4H4), 1,3-butadiene (C4H6),and 1-/2-methyl-1,3-butadiene (C5H8): (6) naphthalene (C10H8; phenyl–vinylacetylene), (7)1-methylnaphthalene (C10H7CH3; meta-tolyl–vinylacetylene), (8) 2-methylnaphthalene (C10H7CH3;meta/para-tolyl–vinylacetylene), (9) 1,4-dihydronaphthalene (C10H10; phenyl–1,3-butadiene), (10)9-methyl-1,4-dihydronaphthalene (C10H9CH3; meta-tolyl–1,3-butadiene), (11)8-methyl-1,4-dihydronaphthalene (C10H9CH3; meta/para-tolyl–1,3-butadiene), (12)1-methyl-1,4-dihydronaphthalene (C10H9CH3; phenyl–1-methyl-1,3-butadiene), and (13)2-methyl-1,4-dihydronaphthalene (C10H9CH3; phenyl–2-methyl-1,3-butadiene) (83, 90–92).

radical (C6D5) with vinylacetylene by accessing the C10H9 potential energy surface (Figures 2,3a, and 4a) (83). For the phenyl-vinylacetylene system, a reactive scattering signal was detectedat m/z = 128 (C10H+

8 ), indicating that the reaction of the phenyl radical with vinylacetyleneleads to the synthesis of a hydrocarbon molecule with the molecular formula C10H8 plus atomichydrogen via a single-collision event. The reaction of the D5-phenyl radical with vinylacetylene


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–98

+

Figure 3(a) Relevant sections of the potential energy surface of the reaction of phenyl radicals (C6H5) withvinylacetylene (C4H4) leading to naphthalene in crossed molecular beam reactions (83). The proposed pathwayleading to naphthalene in cold molecular clouds is highlighted in red. (b) Relevant sections of the potentialenergy surface of the reaction of phenyl radicals with 1,3-butadiene (C4H6) leading to 1,4-dihydronaphthalenein crossed molecular beam reactions (90). The pathway leading to 1,4-dihydronaphthalene in cold molecularclouds is highlighted in red. Relative energies are given in units of kilojoules per mole.


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a

100 ms–1

b

100 ms–1

Figure 4Flux contour plots of the reaction of (a) the phenyl radical with vinylacetylene leading to naphthalene and (b) the phenyl radical with1,3-butadiene forming 1,4-dihydronaphthalene.

was designed to determine if the hydrogen atom is emitted from the vinylacetylene molecule. Areactive scattering signal was observed at m/z = 133 (C10H3D+

5 ), indicating that the hydrogenatom is released from the vinylacetylene reactant. Considering that the count rates of the hydrogenatom loss in the reactions of vinylacetylene with phenyl and D5-phenyl radicals are essentiallyidentical within the error limits, the hydrogen atom is suggested to be ejected predominantly fromthe vinylacetylene reactant and not from the phenyl group. This interpretation of the raw dataalone provided convincing evidence that in the reaction of phenyl radicals with vinylacetylene, ahydrocarbon of the molecular formula C10H8 is formed, with atomic hydrogen loss as the drivingforce and the hydrogen atom originating from the vinylacetylene reactant.

The laboratory data were converted into the center-of-mass frame and combined with elec-tronic structure calculations to identify the product isomer(s) formed (Figure 3a). The high-energy cutoffs of the center-of-mass translational energy distributions revealed reaction energiesof −268 ± 30 kJ mol−1, which are consistent with the formation of at least the naphthaleneproduct. These data agree nicely with the computed data (−265 ± 5 kJ mol−1) and data in theliterature (−276 ± 18 kJ mol−1) to form naphthalene plus atomic hydrogen. We note that the1-phenyl-vinylacetylene isomer (C6H5HCCHCCH) is less stable by 223 ± 5 kJ mol−1 and can-not account for the experimentally derived reaction energy. Furthermore, the P(ET ) distributionshows a pronounced maximum at 30–40 kJ mol−1, indicating the involvement of a tight exit tran-sition state to form naphthalene plus a hydrogen atom. The fact that the center-of-mass angulardistribution T(θ ) displays intensity over the complete angular range demonstrates that the reac-tions follow indirect scattering dynamics through the formation of C10H9 and C10H4D5 complexeswith lifetimes longer than their rotational periods. Most importantly, the T(θ ) distribution hasa pronounced maximum at 90◦, as is evident from the flux contour plots (Figure 4a). This sig-nals sideways scattering and exposes geometrical constraints of the fragmenting C10H9/C10H4D5

intermediates, with the hydrogen atom ejected almost parallel to the total angular momentumvector and hence perpendicular to the molecular plane of the rotating, decomposing complex(es).

These findings, combined with electronic structure calculations, and tracing the atomic hydro-gen loss with deuterated reactants, allow us to identify two open pathways to naphthalene, among


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them one with a de facto barrierless route in the entrance channel. These reactions proceed throughaddition of the phenyl radical to the terminal C1 and C4 carbon atoms of vinylacetylene to formC10H9 intermediates [1] and [2] shown in Figure 3a bound by 187 and 208 kJ mol−1, respectively.Most importantly, when the phenyl radical approaches the C1 carbon atom of vinylacetylene,the potential energy surface is attractive until the formation of a van der Waals complex ([0] inFigure 3a), which is weakly stabilized by 19 kJ mol−1 with respect to the separated reactants.The further approach leading to the formation of a carbon-carbon bond and intermediate [1] ishindered by a barrier of 5 kJ mol−1, but the corresponding transition state lies lower in energythan the separated phenyl plus vinylacetylene reactants. In this sense, a barrier to addition exists,but this barrier is located below the energy of the separated reactants and is called a submergedbarrier. For the overall reaction from phenyl plus vinylacetylene to intermediate [1], the reactionis de facto barrierless. After isomerization to intermediate [1], the latter undergoes a hydrogenshift from the phenyl ring to the vinylacetylene moiety to yield intermediate [3], followed byring closure to intermediate [4] and a further hydrogen shift, forming intermediate [5]. A hydro-gen ejection from the C1 carbon atom of the 1-hydro-naphthalene intermediate (intermediate[5]) leads to naphthalene via a tight exit transition state located 24 kJ mol−1 above the prod-ucts. The competing pathway to naphthalene, via addition to C4, involves an entrance barrier of5 kJ mol−1. The resulting doublet radical (intermediate [2]) undergoes a hydrogen shift followedby cyclization and naphthalene formation (intermediate [6] → intermediate [5] → naphthaleneplus atomic hydrogen). Our computations also revealed that three monocyclic products—cis-1-phenyl-vinylacetylene (C6H5HCCHCCH), 4-phenyl-vinylacetylene (C6H5CCHCCH2), andtrans-1-phenyl-vinylacetylene (C6H5HCCHCCH)—are energetically accessible; these isomers,however, are more than 200 kJ mol−1 less stable than naphthalene.

At zero pressure and collision energies lower than 5 kJ mol−1, our calculations predict the al-most exclusive formation of naphthalene with branching ratios of approximately 99%. Therefore,in cold molecular clouds, where the barrier to addition to C4 cannot be overcome, naphthalenerepresents the sole reaction product, whose formation is initiated by a van der Waals complex ([0] inFigure 3a) and the addition of the phenyl radical to the C1 carbon atom, which holds the vinyl(H2CCH) moiety, through a submerged barrier, forming intermediate [1]. The successive iso-merization sequence intermediate [1] → intermediate [3] → intermediate [4] → intermediate[5] through hydrogen shift → cyclization → hydrogen shift is terminated by the emission of ahydrogen atom and formation of naphthalene through a tight exit transition state. As predictedexperimentally based on the off-zero peaking of the center-of-mass translational energy distri-bution and also theoretically, the exit transition state is rather tight and located approximately24 kJ mol−1 above the separated products. This can be easily rationalized because the reversereaction involves the addition of a hydrogen atom to a closed-shell aromatic hydrocarbon, whichis associated with an entrance barrier. Finally, the experimentally predicted sideways scattering asvisualized in the flux contour plot (Figure 4a) is also verified by electronic structure calculationsdepicting an angle between the plane of the decomposing intermediate and the direction of theatomic hydrogen loss of 81.9o (Figure 5a).

3.2. Dihydronaphthalene

The synthesis of 1,4-dihydronaphthalene (C10H10)—the prototype of a doubly hydrogenatedPAH—was investigated via the bimolecular reactions of the phenyl radical with 1,3-butadieneand D6-1,3-butadiene (C4D6) as well as of the D5-phenyl radical with 2,3-D2-1,3-butadiene(C4H4D2) and 1,1,4,4-D4-1,3-butadiene (C4H2D4) (Figure 2) (90). For the phenyl–1,3-butadiene system, a reactive scattering signal probed at m/z = 130 (C10H+

10) suggested that


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a

81.9˚

b

93.0˚

Figure 5Geometries of the tight exit transition states in the reactions of (a) the phenyl radical with vinylacetyleneforming naphthalene and (b) the phenyl radical with 1,3-butadiene forming 1,4-dihydronaphthalene.

the reaction of the phenyl radical with 1,3-butadiene results in the formation of a hydrocarbonwith the molecular formula C10H10 plus atomic hydrogen via a single-collision event. Forthe phenyl–D6-1,3-butadiene system, a reactive scattering signal was measured at m/z = 136(C10D6H+

4 ); this signal corresponds to hydrogen atom emission from the phenyl ring. Theintensity of the hydrogen loss was 58 ± 15% compared to that in the phenyl–1,3-butadienesystem. These findings suggest that the hydrogen atom is emitted from both the phenyl ring and1,3-butadiene. The hydrogen loss channel in the reaction of partially deuterated 1,3-butadienemolecules with D5-phenyl radicals was monitored to further investigate the extent to whichhydrogen emission originated from the 1,3-butadiene molecule. The results of these reactionsindicated that the hydrogen emission from the CH2 group as recorded by tracing the reactivescattering signal at m/z = 137 (C10H3D+

7 ) in the reaction of 2,3-D2-1,3-butadiene withD5-phenyl accounts for approximately 32 ± 10% of the signal for the hydrogen loss of thephenyl–1,3-butadiene system, thus accounting for 90 ± 25% of the hydrogen detected via thepartially deuterated reactants compared to the fully hydrogenated system. The interpretationof the raw data alone suggests that in the reaction of phenyl radicals with 1,3-butadiene, ahydrocarbon of the molecular formula C10H10 is formed via atomic hydrogen loss; results with(partially) deuterated experiments indicated the formation of at least two structural isomers viahydrogen loss from the phenyl radical and from the CH2 moiety of the 1,3-butadiene reactant.

The laboratory data were again converted into the center-of-mass frame and combined withelectronic structure calculations to ascertain the product isomer(s) formed (Figure 3b). The high-energy cutoffs of the center-of-mass translational energy distributions exposed reaction energies of−120 ± 30 kJ mol−1, which are consistent with the formation of at least 1,4-dihydronaphthalene.These data agree nicely with the computed data (−98 ± 5 kJ mol−1). The trans-1-phenyl-1,3-butadiene, cis-1-phenyl-1,3-butadiene, and 2-phenyl-1,3-butadiene isomers are less stable by 70–80 kJ mol−1 and cannot account for the experimentally derived reaction energies. Moreover, theP(ET ) distribution exhibits a pronounced distribution maximum of 30–40 kJ mol−1. These findingssuggest the existence of at least one reaction channel that holds a tight exit barrier upon the decom-position of the C10H11 intermediate(s). The T(θ ) distribution revealed intensities over the com-plete angular range, suggesting that the reactions follow indirect scattering dynamics through theformation of C10H11 complexes with lifetimes longer than their rotational periods. Furthermore,


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the maximum at 90◦ suggests the existence of geometrical constraints when the reaction interme-diate decomposes (i.e., a preferential hydrogen loss parallel to the total angular momentum vectorand almost perpendicular to the rotational plane of the decomposing intermediate).

These findings are now being merged with electronic structure calculations (Figure 3b). Thecalculations predict that upon the approach of the phenyl radical to 1,3-butadiene toward theterminal C1 atom, the potential energy surface is attractive until the formation of a van der Waalscomplex ([0] in Figure 3b); this complex is stabilized by 25 kJ mol−1 with respect to the sepa-rated reactants. As the carbon-carbon distance continues to decrease, the system proceeds via asubmerged barrier located 3 kJ mol−1 above the complex but 22 kJ mol−1 below the reactants.Hence, for the overall reaction from phenyl plus 1,3-butadiene to form intermediate [1], the re-action is de facto barrierless. The intermediate can lose a hydrogen atom from the attacked C1carbon to form 1-phenyl-trans-1,3-butadiene with an overall exoergicity of 36 kJ mol−1 and withthe hydrogen atom loss transition state residing 15 kJ mol−1 above 1-phenyl-trans-1,3-butadieneplus atomic hydrogen. After the van der Waals complex [0] isomerizes to intermediate [1], thelatter undergoes a trans-cis isomerization to intermediate [2] followed by ring closure to inter-mediate [3]. Hydrogen ejection from the bridged carbon atom of intermediate [3] leads to the1,4-dihydronaphthalene product via a tight exit transition state located 31 kJ mol−1 above theseparated products. Additionally, through a barrier of 12 kJ mol−1, the phenyl radical can attackthe central carbon atom of 1,3-butadiene, leading to intermediate [4]. Intermediate [4] isomer-izes via phenyl group migration to intermediate [1] or ejects a hydrogen atom to the non-PAHisomers. At the low temperature of interstellar clouds of 10 K, only the barrierless addition to C1forming intermediate [1] is open, whereas the addition to C2 yielding intermediate [4] is closed.Furthermore, the sideways scattering, as verified by a pronounced maximum of the center-of-massangular distribution at 90◦ (Figure 4b), indicated the existence of geometrical constraints upondecomposition of intermediate [3] (i.e., the emission of the hydrogen atom perpendicular to therotation plane of the fragmenting intermediate). This has also been authenticated theoretically,and the computations depict an angle of the hydrogen atom loss of 93.0◦ with respect to themolecular plane (Figure 5b). Finally, the product branching ratios computed at zero pressure andcollision energies lower than 12 kJ mol−1 reveal that 1,4-dihydronaphthalene clearly dominatesthe isomer distribution up to approximately 90%.

3.3. (Di)Methyl-Substituted (Dihydro)Naphthalenes

We now transfer the concepts learned from the barrierless formation of two of the simplestrepresentatives of (hydrogenated) PAHs—naphthalene and 1,4-dihydronaphthalene—to methyl-and dimethyl-substituted (dehydrogenated) PAHs, whose structures are compiled in Figures 2and 6, respectively. In the case of naphthalene, a formal replacement of a single hydrogen atomby a methyl group at the C1 and C2 carbon atoms yields 1- and 2-methylnaphthalene (molecules7 and 8 in Figure 2, respectively). Based on the concept of the chemical reactivity of the phenylradical with vinylacetylene (Section 3.1), we might foresee that a replacement of a hydrogenatom in the phenyl radical reactant by a methyl group should not have any effect on the reactionmechanisms (Figure 3). Hence, if the methyl group is replaced at a carbon atom in the phenylradical far away from the radical center, such as in the meta and para position, yielding meta- andpara-tolyl, the methyl group should act solely as a spectator without inducing sterical hindrancein the addition process and isomerization via hydrogen migration and ring closure. The resultsof the crossed molecular beam reactions of vinylacetylene with meta-tolyl (92) and para-tolyl (91)confirm these predictions. Approaching the C1 atom holding the H2C moiety of vinylacetylene,both the meta- and para-tolyl radicals form van der Waals complexes, which are stabilized by


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approximately 5 kJ mol−1 with respect to the reactants. The complexes lead to the formationof carbon-carbon bonds once again through submerged barriers located only 1 kJ mol−1 abovethe van der Waals complex but 4 kJ mol−1 below the energy of the reactants. These pathwayslead to the formation of methyl-substituted complexes (intermediate [1] in Figure 3), which thenundergo isomerization via hydrogen shift → cyclization → hydrogen shift and hydrogen emission,forming 1- and 2-methylnaphthalenes via tight exit transition states. In cold molecular clouds, thecompeting additions to C4 have barriers of ∼7 kJ mol−1 and hence are closed. In analogy to thereaction of vinylacetylene with the phenyl radical, both the meta- and para-tolyl radical reactionsform methylnaphthalenes with yields of 99% at collision energies of less than 7 kJ mol−1 (i.e.,lower than the barrier to addition at the C4 carbon atom). It is important to note that because ofthe molecular structure of the radical reactant, the reaction of the para-tolyl radical leads solelyto 2-methylnaphthalene, whereas the meta-tolyl radical forms 1- and 2-methylnaphthalene uponreaction with vinylacetylene. Here, the hydrogen shift in the methyl-substituted counterpart ofintermediate [1] can take place from the C2 and C5 carbon atoms of the phenyl moiety to theC2 atom of the vinylacetylene moiety, resulting in the formation of two chemically nonequivalentmethyl-substituted intermediate [3] types, which in turn undergo ring closure to distinct methyl-substituted intermediate [4] types. However, in the case of the para-tolyl–vinylacetylene system,both the C2 and C5 positions of the phenyl moiety are chemically equivalent, and only a singlemethyl-substituted intermediate [3] is formed. In summary, the reaction of meta- and para-tolylradicals with vinylacetylene follows the pattern of the barrierless aromatization in the phenyl-vinylacetylene system, leading almost exclusively to 1- and 2-methylnaphthalene under conditionsresembling cold molecular clouds.

Having transferred the concept of barrierless ring-closure reactions leading to aromatizationfrom naphthalene to methyl-substituted naphthalenes, we can now shift the principle in a sim-ilar pattern from 1,4-dihydronaphthalene to its (di)methyl-substituted counterparts (Figures 2and 6). In the case of 1,4-dihydronaphthalene, the situation is more complex because, comparedto naphthalene, both rings are chemically nonequivalent. This can lead to the replacement ofa hydrogen atom at the aromatic ring at the C6 and C7 positions, resulting in 9-methyl-1,4-dihydronaphthalene (C10H9CH3; molecule 10 in Figure 2) and 8-methyl-1,4-dihydronaphthalene(C10H9CH3; molecule 11), or at the olefinic ring, yielding 1-methyl-1,4-dihydronaphthalene(C10H9CH3; molecule 12) and 2-methyl-1,4-dihydronaphthalene (C10H9CH3; molecule 13), re-spectively. Considering that the reaction of the phenyl radical with 1,3-butadiene synthesized the1,4-dihydronaphthalene molecule (Section 3.2), a hydrogen atom can be replaced in the phenylring at the meta or para positions (i.e., reactions of meta- and para-tolyl with 1,3-butadiene, form-ing molecules 10 and 11). Alternatively, a hydrogen atom can be substituted by a methyl groupat the C1 and C2 positions of 1,3-butadiene, resulting in reactions of 1-methyl-1,3-butadieneand 2-methyl-1,3-butadiene with phenyl radicals, forming molecules 12 and 13, respectively. Alladditions to the C1 carbon atom involve the initial formation of a van der Waals complex, whichis bound by approximately 7 kJ mol−1 with respect to the reactants. After passing a submergedbarrier to addition, which is located 4–5 kJ mol−1 below the energy of the separated reactants,methyl-substituted intermediate [1] is formed, which then undergoes trans-cis isomerization, ringclosure, and hydrogen loss through tight exit transition states located 29–40 kJ mol−1 above theproducts, yielding methyl-substituted 1,4-dihydronaphthalenes (molecules 10–13).

Finally, we explore the reaction mechanisms leading to dimethyl-substituted 1,4-dihydronaphthalenes (Figure 6). The replacement of two hydrogen atoms by a methyl groupat each ring (aromatic versus olefinic) requires crossed beam experiments in which both reactantshold a methyl group. Both the meta- and para-tolyl radical reactants fulfill this requirement forthe radical reactant, as do the 1- and 2-methyl-1,3-butadiene hydrocarbon reactants. Replacing


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CH3CH3

21

CH3

CH3

20

CH3

H3C

17

H3C

CH3

16

CH3

CH3

19

CH3

CH3

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H3C CH3

14

Figure 6Dimethyl-substituted 1,4-dihydronaphthalene isomers formed in exoergic and de facto barrierlessbimolecular reactions of meta- and para-tolyl radicals with 1-methyl-1,3-butadiene and2-methyl-1,3-butadiene under single-collision conditions (92).

the hydrogen atoms in the phenyl radical and in the 1,3-butadiene molecule pairwise (Figure 3),and tracing the methyl groups in the reaction, leads to the formation of eight distinct dimethyl-substituted 1,4-dihydronaphthalene molecules, as compiled in Figure 6. Once again, the ad-dition to the C1 carbon atom of the methyl-substituted 1,3-butadiene reactant is preceded bythe formation of a van der Waals complex, which then isomerizes via a submerged barrier todimethyl-substituted reaction intermediates. Upon trans-cis isomerization and cyclization, a ter-minal hydrogen atom loss through a tight transition state results in the formation of discretedimethyl-substituted 1,4-dihydronaphthalene isomers.


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4. DISCUSSION

Above we demonstrate that de facto barrierless entrance and overall exoergic gas phase reactions ofphenyl-type radicals with vinylacetylene and (methyl-substituted) 1,3-butadiene conducted undersingle-collision conditions lead to the formation of (methyl-substituted) naphthalene and 1,4-dihydronaphthalene. Here we attempt to extract generalized concepts on the chemical reactivityof phenyl-type radicals leading to PAH formation at ultralow temperature conditions in coldmolecular clouds of typically 10 K.

All bimolecular reactions of phenyl-type radicals with the C4 and C5 hydrocarbons vinylacety-lene and (methyl-substituted) 1,3-butadiene leading to PAH-type products as discussed above aredictated by attractive long-range interaction forces and initiated by the formation of van der Waalscomplexes, which are weakly bound by 7–25 kJ mol−1 with respect to the separated reactants. PAHformation commences by the addition of the phenyl-type radicals to the C1 carbon atom holdingthe H2C moiety of the vinyl group. The barrier to addition is typically 1–5 kJ mol−1 above thevan der Waals complex but is below the energy of the separated reactants. Therefore, a barrier toaddition does exist, but it is located below the energy of the reactants (submerged barrier), and thereaction proceeds de facto without any entrance barrier. This finding deserves further discussion.Conventional wisdom dictates that the addition of phenyl-type radicals to an olefinic double bondis associated with entrance barriers, for example, 10 kJ mol−1 for ethylene as one of the buildingblocks of vinylacetylene and 1,3-butadiene. Even though this barrier is quite low, it cannot beovercome at low temperatures of 10 K in cold molecular clouds. However, the enhanced polar-izabilities of the C4 and C5 hydrocarbons vinylacetylene and (methyl-substituted) 1,3-butadiene,ranging from 7.7 A3 to 10.0 A3, correlate with greater attractive long-range dispersion forcesbetween the phenyl-type radical and the C4/C5 hydrocarbon compared to ethylene (4.15 A3).The enhanced polarizabilities and attractive long-range dispersion forces trigger the formationof a weakly bound van der Waals complex ([0] in Figure 3), which subsequently isomerizes viaa submerged barrier. The required existence of a submerged barrier to addition necessitates thatthe phenyl-type radical must react with a C4 or higher hydrocarbon, which in turn must carry avinyl group in order to form a weakly bound van der Waals complex. The van der Waals complexsubsequently isomerizes via the addition of the phenyl-type radical with its radical center to theH2C moiety of the vinyl group, forming doublet radicals, which are bound by 180–200 kJ mol−1.

These acyclic doublet radicals undergo multiple rearrangements. In the case of the vinylacety-lene reactant, a hydrogen shift from the phenyl-type group to the C2 carbon atom of the viny-lacetylene moiety is followed by ring closure and another hydrogen shift, effectively leading tothe formation of (methyl-substituted) singly hydrogenated naphthalene intermediates, which theneject a hydrogen atom from C1, forming (methyl-substituted) naphthalenes. The emitted hydro-gen atoms originate solely from the H2C moiety of the vinyl-type group of the vinylacetylenereactant. Considering the (methyl-substituted) 1,3-butadiene reactants, a trans-cis isomerizationin the addition complex is followed by ring closure, forming (methyl-substituted) singly hydro-genated 1,4-dihydronaphthalene intermediates, which then lose a hydrogen atom from the bridgecarbon atom, forming (methyl-substituted) 1,4-dihydronaphthalenes. It is important to highlightthat the ejected hydrogen atoms originate solely from the phenyl-type radical and that all exittransition states involving atomic hydrogen loss and aromatization are rather tight and located upto 40 kJ mol−1 above the separated products. Furthermore, in each system, the hydrogen atomis lost almost perpendicularly to the plane of the decomposing complex (Figures 4 and 5). Thissideways scattering can be rationalized by considering the molecular structures of the naphthaleneand 1,4-dihydronaphthalene products. Here, in the reversed reaction, the hydrogen atom has tointeract with the π-electron system of the aromatic moieties, which are located perpendicular to


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the molecular plane of naphthalene and 1,4-dihydronaphthalene. Finally, at zero pressure and zerocollision energy—conditions replicating gas phase conditions of cold molecular clouds—the for-mation of the PAH-type products is almost quantitative, with typical branching ratios of 99%, withthe remaining 1% leading to the formation of non-PAH products, which are thermodynamicallyless stable typically by 200 and 90 kJ mol−1 for the naphthalene- and 1,4-dihydronaphthalene-typesystems, respectively.

We are now testing these generalized concepts of de facto barrierless and exoergic PAH forma-tion via reactions of phenyl-type radicals in an attempt to derive additional prerequisites to PAHformation. First, we tested the prerequisite of a C4 hydrocarbon reacting with a phenyl-type radicalfor the reaction to proceed barrierlessly. Our studies of the phenyl-propene (C3H6; H2CCHCH3)(93) system and of the reactions of phenyl-type radicals (phenyl, meta-tolyl, para-tolyl) (92, 94)with two C3H4 isomers, allene (H2CCCH2) and methylacetylene, revealed that these reactionsinvolve barriers to addition of 6–14 kJ mol−1. The absence of van der Waals complexes mightbe attributed to the lower polarizabilities of the C3 reactants of only 6.2–6.3 A3 compared toup to 10.0 A3 of the C4 reactants. In the case of the C3H4 isomers allene and methylacetylene,both reactions ultimately led to the formation of (methyl-substituted) indene isomers, with indeneconsidered as the prototypical PAH carrying one six- and one five-membered ring (Figure 7),however, only after passing an initial entrance barrier to the addition of the phenyl-type radical.Therefore, reactions of phenyl-type radicals with methylacetylene and allene cannot form PAHs

22

H3C

H3C

23 24

CH3

CH3

25 26

Figure 7Indene and methyl-substituted indene isomers formed in exoergic reactions involving entrance barriers ofphenyl-type radicals (phenyl and meta- and para-tolyl) with allene and methylacetylene under single-collision conditions (92, 94).


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in cold molecular clouds but potentially can in circumstellar envelopes of carbon stars, where el-evated temperatures of a few thousand Kelvin help to overcome the barriers to addition. Second,we are investigating the effect of enhanced hydrogen content in the hydrocarbon reactant. Withrespect to the propene reactant, the propene-phenyl reaction did not lead to the formation ofPAH-type products (93, 95). Detailed electronic structure calculations revealed that the initialaddition intermediate preferentially decomposed via atomic hydrogen and methyl loss pathways,forming non-PAH products; the synthesis of the PAH derivative indane (C9H10) would requirean addition → hydrogen shift → cyclization → hydrogen loss sequence, in which the transitionstate for the hydrogen shift is located almost 20 kJ mol−1 above the exit barrier to lose a hy-drogen atom from the initial collision complex and approximately 9 kJ mol−1 above the energyof the separated reactants, thus effectively blocking the formation of indane. The energeticallyunfavorable nature of this transition state can be readily understood considering that, owing tothe enhanced hydrogen content in the hydrocarbon reactant, the resulting doublet radical lacksany resonance stabilization, thus making hydrogen migration energetically less favorable than inthe case of indene (C9H8) formation. The failed PAH formation in the crossed beam reaction ofbutene (C4H8) with phenyl radicals (95) follows this trend. Conversely, the reaction of phenyl-typeradicals with the hydrogen-poor reactants vinylacetylene and 1,3-butadiene involves the formationof RSFR intermediates and, inherently, relatively low-lying atomic hydrogen migration and/orcyclization transition states. However, the enhanced hydrogen content of the butene reactantprohibits the formation of any RSFR intermediate and ultimately PAH formation. Therefore,these considerations suggest that RSFR intermediates and inherently lower barriers to their iso-merization reactions, as compared to those for immediate atomic hydrogen loss or carbon-carbonbond cleavage, are crucial to the synthesis of PAH products; enhanced hydrogen content in thereactants inhibits the formation of RSFRs, eventually blocking PAH synthesis.

Finally, we discuss the effect of the molecular structure of the hydrocarbon reactant onPAH formation by comparing the reactivity of four butadiene isomers: 1,3-butadiene, 1-butyne(HCCC2H5) (89), 2-butyne (CH3CCCH3) (89), and 1,2-butadiene [H2CCCH(CH3)] (96). Com-bined crossed beam and computational studies revealed that only the reaction of 1,3-butadiene withphenyl (Section 3.2) leads to PAH formation. Reactions of phenyl radicals with the other isomersare dictated by a phenyl radical addition followed by either carbon-carbon or carbon-hydrogenbond rupture processes, leading to non-PAH products. Considering the molecular structures of1-butyne, 2-butyne, and even 1,2-butadiene, it is evident that the initial addition product hasto undergo multiple hydrogen shifts prior to ring closure and hydrogen loss. The lack of for-mation of resonantly stabilized radical intermediates in these hydrogen migrations again resultsin energetically unfavorable isomerization steps, which cannot compete with carbon-hydrogen orcarbon-carbon bond rupture processes. However, in the case of vinylacetylene (H2CCHCCH) and(methyl-substituted) 1,3-butadiene, resonantly stabilized reaction intermediates can be formedowing to the presence of –C≡CH and –HC=CH2 groups in conjugation with the vinyl-typegroup (H2CCR; R = H, CH3), to which the phenyl-type radical is added via a submerged bar-rier. Therefore, the presence of –C≡CH and –HC=CH2 groups in conjugation with a vinyl-typegroup (H2CCR; R = H, CH3), to which the phenyl-type radical is added via a submerged barrier,in a hydrocarbon reactant larger than C3 is a crucial prerequisite to PAH formation.

5. CONCLUSIONS AND OUTLOOK

Combined crossed beam and electronic structure calculations revealed the facile for-mation of (methyl-substituted) PAHs, which can be formally derived from naphthaleneand 1,4-dihydronaphthalene, upon reaction of phenyl-type radicals with vinylacetylene and


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hνhν hνhν

Figure 8Schematic representation of the stepwise growth of polycyclic aromatic hydrocarbons in the interstellar medium via reactions ofphenyl-type radicals with vinylacetylene involving low-temperature chemistry.

(methyl-substituted) 1,3-butadiene. The underlying reaction dynamics involve a de facto bar-rierless addition of the phenyl-type radical within an initial van der Waals complex througha submerged barrier to a vinyl-type moiety, which must be in conjugation with a –C≡CH or–HC=CH2 group in the hydrocarbon reactant to form an RSFR intermediate. These intermedi-ates then rearrange, ultimately forming a cyclic intermediate, which then undergoes aromatization


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and PAH formation via atomic hydrogen loss. For the formation of a van der Waals complex, suf-ficiently strong, attractive, long-range interactions have to be present, suggesting that phenyl-typeradicals only form van der Waals complexes with C4 or higher hydrocarbons. In cold molecu-lar clouds, these de facto entrance barrierless, elementary reactions form naphthalene- and 1,4-dihydronaphthalene-based PAHs in yields of up to 99%, also demonstrating the unique connectionbetween RSFR intermediates and PAH formation.

These results are also vital to predict synthetic routes to form more complex PAHs, such asanthracene and phenanthrene, in the ISM, considering that a recent computational study re-vealed the HACA mechanism to be inefficient in the formation of PAHs beyond acenaphtha-lene (35). In the ISM, via atomic hydrogen loss from the C1 and C2 positions, the photodis-sociation of naphthalene can lead to 1- and 2-naphthyl radicals (Figure 8) (83). Reactions ofthe latter with vinylacetylene might propagate PAH extension to systems with three fused ben-zene rings: anthracene and phenanthrene. These reactions are also expected to be barrierlessowing to the much stronger bound van der Waals complex in the entrance channel betweenvinylacetylene and the naphthyl radical, holding polarizabilities of 7.7 A3 and 16.5 A3, respec-tively. Therefore, we might classify the reaction of vinylacetylene with the phenyl radical as theprototype of a class of vinylacetylene-mediated reactions for PAH growth in cold interstellarenvironments.

To conclude, the simple route to barrierless PAH formation in low-temperature environ-ments via bimolecular reactions involving phenyl-type radicals presents a fundamental shift frompreviously accepted perceptions of PAH formation via HACA-based pathways and suggests thatcold molecular clouds such as TMC-1 and OMC-1 are potential molecular nurseries of PAHsynthesis. This view challenges conventional theories that PAHs can be synthesized only in high-temperature environments, such as those present in circumstellar envelopes of evolved carbonstars such as IRC+10216. Therefore, the edges of molecular clouds, at which atomic gas is trans-formed into molecular gas and carbon drives a complex hydrocarbon chemistry, are likely theprime growth locations of PAH formation via de facto barrierless routes involving two neutralreactants.

DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings thatmight be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS

This work was supported by the US Department of Energy, Basic Energy Sciences, via grants DE-FG02-03ER15411 to R.I.K. at the University of Hawaii and DE-FG02-04ER15570 to A.M.M.at Florida International University.

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94. Parker DSN, Zhang F, Kaiser RI, Kislov VV, Mebel AM. 2011. Indene formation under single-collisionconditions from the reaction of phenyl radicals with allene and methylacetylene: a crossed molecular beamand ab initio study. Chem. Asian J. 6:3035–47

95. Albert DR, Todt MA, Davis HF. 2013. Crossed molecular beams studies of phenyl radical reactions withpropene and trans-2-butene. J. Phys. Chem. A 117:13967–75

96. Yang T, Parker DSN, Dangi BB, Kaiser RI, Kislov VV, Mebel AM. 2014. Crossed beam reactions of thephenyl (C6H5; X2A1) and phenyl-d5 radical (C6D5; X2A1) with 1,2-butadiene (H2CCCHCH3; X1A′).J. Phys. Chem. A 118:6181–90


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PC66-FrontMatter ARI 4 March 2015 12:22

Annual Review ofPhysical Chemistry

Volume 66, 2015Contents

Molecules in Motion: Chemical Reaction and Allied Dynamics inSolution and ElsewhereJames T. Hynes � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1

Crystal Structure and PredictionTejender S. Thakur, Ritesh Dubey, and Gautam R. Desiraju � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �21

Reaction Dynamics in Astrochemistry: Low-Temperature Pathways toPolycyclic Aromatic Hydrocarbons in the Interstellar MediumRalf I. Kaiser, Dorian S.N. Parker, and Alexander M. Mebel � � � � � � � � � � � � � � � � � � � � � � � � � � � � �43

Coherence in Energy Transfer and PhotosynthesisAurelia Chenu and Gregory D. Scholes � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �69

Ultrafast Dynamics of Electrons in AmmoniaPeter Vohringer � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �97

Dynamics of Bimolecular Reactions in SolutionAndrew J. Orr-Ewing � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 119

The Statistical Mechanics of Dynamic Pathways to Self-AssemblyStephen Whitelam and Robert L. Jack � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 143

Reaction Dynamics at Liquid InterfacesIlan Benjamin � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 165

Quantitative Sum-Frequency Generation Vibrational Spectroscopy ofMolecular Surfaces and Interfaces: Lineshape, Polarization,and OrientationHong-Fei Wang, Luis Velarde, Wei Gan, and Li Fu � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 189

Mechanisms of Virus AssemblyJason D. Perlmutter and Michael F. Hagan � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 217

Cold and Controlled Molecular Beams: Production and ApplicationsJustin Jankunas and Andreas Osterwalder � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 241

Spintronics and Chirality: Spin Selectivity in Electron TransportThrough Chiral MoleculesRon Naaman and David H. Waldeck � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 263

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DFT: A Theory Full of Holes?Aurora Pribram-Jones, David A. Gross, and Kieron Burke � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 283

Theoretical Description of Structural and Electronic Properties ofOrganic Photovoltaic MaterialsAndriy Zhugayevych and Sergei Tretiak � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 305

Advanced Physical Chemistry of Carbon NanotubesJun Li and Gaind P. Pandey � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 331

Site-Specific Infrared Probes of ProteinsJianqiang Ma, Ileana M. Pazos, Wenkai Zhang, Robert M. Culik, and Feng Gai � � � � � 357

Biomolecular Damage Induced by Ionizing Radiation: The Direct andIndirect Effects of Low-Energy Electrons on DNAElahe Alizadeh, Thomas M. Orlando, and Leon Sanche � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 379

The Dynamics of Molecular Interactions and Chemical Reactions atMetal Surfaces: Testing the Foundations of TheoryKai Golibrzuch, Nils Bartels, Daniel J. Auerbach, and Alec M. Wodtke � � � � � � � � � � � � � � � � 399

Molecular Force Spectroscopy on CellsBaoyu Liu, Wei Chen, and Cheng Zhu � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 427

Mass Spectrometry of Protein Complexes: From Originsto ApplicationsShahid Mehmood, Timothy M. Allison, and Carol V. Robinson � � � � � � � � � � � � � � � � � � � � � � � � � � 453

Low-Temperature Kinetics and Dynamics with Coulomb CrystalsBrianna R. Heazlewood and Timothy P. Softley � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 475

Early Events of DNA PhotodamageWolfgang J. Schreier, Peter Gilch, and Wolfgang Zinth � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 497

Physical Chemistry of Nanomedicine: Understanding the ComplexBehaviors of Nanoparticles in VivoLucas A. Lane, Ximei Qian, Andrew M. Smith, and Shuming Nie � � � � � � � � � � � � � � � � � � � � � 521

Time-Domain Ab Initio Modeling of Photoinduced Dynamics atNanoscale InterfacesLinjun Wang, Run Long, and Oleg V. Prezhdo � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 549

Toward Design Rules of Directional Janus Colloidal AssemblyJie Zhang, Erik Luijten, and Steve Granick � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 581

Charge Transfer–Mediated Singlet FissionN. Monahan and X.-Y. Zhu � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 601

Upconversion of Rare Earth NanomaterialsLing-Dong Sun, Hao Dong, Pei-Zhi Zhang, and Chun-Hua Yan � � � � � � � � � � � � � � � � � � � � � � 619

vi Contents

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Computational Studies of Protein Aggregation: Methods andApplicationsAlex Morriss-Andrews and Joan-Emma Shea � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 643

Experimental Implementations of Two-Dimensional FourierTransform Electronic SpectroscopyFranklin D. Fuller and Jennifer P. Ogilvie � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 667

Electron Transfer Mechanisms of DNA Repair by PhotolyaseDongping Zhong � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 691

Vibrational Energy Transport in Molecules Studied byRelaxation-Assisted Two-Dimensional Infrared SpectroscopyNatalia I. Rubtsova and Igor V. Rubtsov � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 717

Indexes

Cumulative Index of Contributing Authors, Volumes 62–66 � � � � � � � � � � � � � � � � � � � � � � � � � � � 739

Cumulative Index of Article Titles, Volumes 62–66 � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 743

Errata

An online log of corrections to Annual Review of Physical Chemistry articles may befound at http://www.annualreviews.org/errata/physchem

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